The invention relates to a method for testing features of containers which are sealed by a closure, mechanical vibrations being excited in the closure and the mechanical vibrations being analysed. These feature may be for example the central fit of the closure on the container opening, the tightness of the attachment of the closure to the container opening or the residual air volume in containers in which the liquid is foamed up prior to closure in order to expel the residual air or which are to be filled to the rim for process-related reasons.
It is known from DE-A-40 04 965 to test below-atmospheric pressure closures with a spring-return top, in particular below-atmospheric pressure screw-type caps, for the tightness of their attachment by creating mechanical vibrations in the top, processing and evaluating the vibrations as regards their frequency, duration, time integral of the amplitude and/or attenuation and ascertaining the level of the below-atmospheric pressure in the container from this. The below-atmospheric pressure can for example be produced by blowing steam into the remaining residual volume of the container or by pouring the drink in hot. In each case, the below-atmospheric pressure develops only after the steam has cooled or when the drink has assumed the ambient temperature. Similar methods for measuring the tightness of the closure are also known for containers, in which the contents are subjected to above-atmospheric pressure, in particular drinks bottles with carbonated drinks such as mineral waters or beer. This above-atmospheric pressure also builds up only gradually. Testing the tightness of container closures indirectly by testing the above-atmospheric pressure or below-atmospheric pressure in a container is thus possible only if the below-atmospheric pressure has formed through temperature equalization or a degree of above-atmospheric pressure has formed through escaping carbonic dioxide. Immediately after the closures have been fitted by a closer, it is therefore necessary to wait for a certain time before the tightness test can be carried out. The containers are transported further for approx 8 m on a conveyor at a speed of approximately 0.5 m/s to 1.5 m/s during this time; assuming that a noticeable rise or drop in pressure arises only after 5-10 seconds at the earliest (even later for hot pouring), testing via differences in pressure is possible, according to the sensitivity of the measurement apparatus only from 5 to 10 meters behind the closer. In particular, because not only is the rise/drop in pressure small, but in addition the leak is extremely small.
When filling foaming drinks, in particular beer, care must be paid to enclose as little air as possible in the bottle, as the taste of the beer is adversely effected by the atmospheric oxygen. There is no possibility of continuously measuring the level of the enclosed air volume. At present, the level of air volume can only be checked by random spot checks by opening the beer bottle.